Indian Journal of Radi o & Space Physics
Vol. 3 I, August 2002, pp. 190-196
Solar causes of geomagnetic storms
Santosh Kumar
Department of PG Studies & Research in Physics and Electronics, R D University, Jabalpur (MP) 4 82 001
and
Mahendra Pratap Yadav
Govt. Tilak PG College, Kat ni (MP) 483 501
Received 7 Jun e 2001; revised 27 September 2001; accepted 18 January 2002
Two hund red ninet y nine geomagnetic storms (GMSs) o f moderate, moderate ly severe and severe type with Ar ~ 20
have been investi gated for the period 1978-99 and thei r possible interpl anetary and solar cau ses are looked upon. Solar
features like Ha, X-ray solar flare s and active prominences and di sappearin g filaments (APDFs) have been found to occ ur
more in lower hel iog raphi c latitudinal zo nes and produce larger number of GMSs at various locati ons on th e ea rth. No
signifi ca nt corre lati on between mag nitude o f GMSs and importance of Ha and X-ray solar fl ares and APDFs has been
obse rved. Maximum number of GMSs is associated with importance SF of eac h Hu an d X-ray solar fl ares. On the basis of
the statisti ca l sa mpling of fo ur putati ve solar causes o f GMSs on th e earth , th e causes are identifi ed lead ing to magnetic
disturbance with Ar ~ 20. The Ha, X-ray solar flares are more plausible ca uses of GMSs. However. a ver) few GMSs have
been observed wi th out be ing associa ted wit h either of th ese four solar fem ures. The occ urrence o f di sturbed day s varies
with seasons. Further, monthl y di stributi on of di sturbed day s shows a cyclic va riati o n hav ing two peah during the solar
cyc le.
1 Introduction
Geomagne tic
disturbances
are
generall y
represented by geomagnet ic storms and sudde n
ionospheric disturbances (SIDs). These can be
di stingu ished in two categori es which are originated
1
from two types of sol ar wind streams • Th e first
category of geomagnetic sto rms, know n as gradu al
storm
commencement
(GSC),
arises
from
magneticall y open, lo ng lived, hi gh speed so lar w ind
streams (HSSWS) emitted by solar coronal ho les and
is usuall y small in magnitude and exhi bits an apparent
te ndency to occur w ith 27 days rotat ion period of the
sun 2• The second category of geomagnetic storms is
associated with flare generated stream, kn own as
sudden storm com me ncement (SSC) o r sudden
impul ses (S l) . This is relativel y large in magnitude
and arises from the tran sient erupti on of closed field
so lar reg ions and it is identified as abrupt ch ange in
ge neral level of geomagneti c field which may be as
large as several per cent of th e undi stu rbed value
measured at th e surface. Thus, SSCs are caused by
shock waves propagatin g throug h the interp la netary
medium from the sun . Subsequen tly , Sonett et a/. 3
reported from direct interplanetary observation s that a
shoc k like d iscontinuity is moving from th e sun and is
associated with SSC. Burlaga and Ogi lvie-! have
di scussed some of the events associated w ith shoc ks
for which both plasma and mag netic field data are
available. Nishida 5 suggested that SSC could also be
produced by a non-shock mode, pres umabl y, a
hydromagnetic wave or a tangentiJI di sco ntinuity, and
th e non -shock mode disco ntinuity may be the cause of
Sl. Sonett and Colburn 6 proposed th at SI is due to a
7
reverse shock. Gos ling et a /. , based on inte rplanetary
observations, have estab li shed that an Sl mi ght be
caused by a no n-shock mode discontinuity. Og il vie et
a/. 8 observed a discontinuous dec rease in de nsity and
di scontinuous increase in magnet ic field inte nsity and
no appreciable change in temperature, showin g that
the negative impul ses (Sr) are caused by
hydromagnetic di scontinuity w hose s ignature is that
of a tangent ial discontinuity, and the positive
impul ses (Sn are, some times, caused by
hydromag netic discontinuiti es .
The so lar fl m·e is a most spec tacu lar short-lived
phenomeno n th at occurs on th so lar di sc and is
responsibl e fo r so lar energetic parti cle (SEP) evenl
and geomagnetic storm. Sol r flares transforrr
magnetic ene rgy into several form s. Large so lar flare ~
occ ur in magneticall y complex region, where the fi elc
SANTOSH KUMAR & YADA V : SOLAR CAUSES OF GEOMAGNETIC STORMS
is often strongly sheared. The mechani sm of energy
release is probably associated with magnetic
reconnection. There are two basic phenomena that
occur during mag netic reconnection in the flare site.
One of them is rapid heating of coronal and
chromos pheric material, which expands outward into
interplanetary medium and produces interplanetary
(IP) shocks which cause geomagnetic storms and
auroras. The other phenomenon is associated with
particle acceleration, which represents the energy
aspect of the flares. Many workers have shown the
association of di fferent types of geomagnetic storms
with solar fl ares 9 · 10 .
The occurrences of flares and prominences are
associated with varying phases of sunspot cycle. The
solar output in terms of particle and fi eld ejected out
into
interplanetary
medium
influences
the
geomagnetic field conditions 11• The radi ant
electromagneti c and corpuscular radiation produce
extra ioni zation in the sun-lit part of the earth and
produces disturbances at various locations, such as,
polar midlatitude and eq uatorial regions. These
geomagnetic disturbances are observed and
represented by different geo magnetic indices 12 like
AE, KP or Ap and eq uatorial index Ds1. The product of
solar wind velocity (V) and interpl anetary magnetic
fi eld (B), i.e. VB is more important for both cosmic
ray and geo magnetic activity modulations rather than
th e IMF aloneu.
The solar disappearing filaments (DFs) have been
linked with geo magnetic activit/ 4· 15 and also IP
shocks 16• 18 . The CMEs related shocks accelerate so lar
energetic particl es (SE Ps) events associated with
major IP di stu rbances or shocks causing large
geo magnetic stonns 19 . An impo rtant paradigm sh ift,
such that th e CMEs and not flares are considered to
be the key causal link between the solar activity and
all large geomagnetic storms and the associated
effects such as auroral displays, has been reported by
Webb 11• The CMEs are vast structures of solar plasma
and magnetic fields which are expelled from the sun
into the heliosphere and make a prime link between
solar and geomagnetic acti vity. The north-south
component (BJ of IMF, plays a dominan t role in
determinin g th e amount of solar wind energy to be
trans(.errecI to tI1e magnetos pI1crc I · ~()
- . When I 1F has
large magn itude (2: 10 nT) and large so uthward
component, the amo unt of tran sferred energy becomes
very large. On th e other hand , th e tran sferred energy
becomes very small when TMF is directed preli min ary
19 1
northward. The energy transfer efficiency is of the
order of I 0% during intense magnetic storms21 .
Viscous interaction, the other prime energy transfer
mechanism proposed21 , has been shown to be onl y
< I o/o efficient during intense northward directed
IMFs. Tsurutani et a/.22 have examined the
interplanetary and solar causes of five largest
geomagnetic storms during the period 1971-1 986 and
found th at the extreme values of the southward IMF
(BJ rather than th e solar wind speeds are the primary
causes of great magnetic storms. In this paper, an
attempt has been made to examine the effects of solar
and interplanetary transients that may cause GMS .
Further, the GMS and their association with various
phases of sunspot cycles and different interplanetary
and solar features have been investigated for the
period of study .
2 Data analysis
All those geomagnetic storms which are associated
with Ar2:20 during the peri od Jan. 1978- Dec. 1999 are
considered and are found to be 299 in number. For
thi s study, solar wind pl asma (S WP) and
interplanetary magnetic field (IMF) data from IMP-8
satellite are used. These data are compi led by King
and Papitashvili 23 in different volumes of
lnterplalletary Medium Data Book from National
Space Science Data Centre (NSSDC). The possibl e
assoc iation of geomag neti c storms has been
investigated during the peri od Jan. 1978- Dec. 1993.
For assoc iat ion of geo magnetic storms with solar
feat ures, solar geophysical data 24 (SG D) have been
used fo r the period 1978-99. On the basis of so lar
wi nd velocity (V), so lar features have been
investigated such that I :::; !1t :::; 5 days pri or to the
occurrence of GMS on the earth. Here the time (!1t)
taken by the so lar wi nd in reaching th e earth from the
sun will depend upon the solar wind velocity (V) .
Further, the occurrence frequency of geo magnetic
storms and disturbed days with sunspot cycle is
cons idered for the entire period of considerati on, i.e.,
1978-99.
3 Results a nd discussion
The geo magnetic storms with planetary index Ap 2:
:w have been investigated for th e peri od 1978-99
using solar geo phys ical data reports. The
class ification of th e se lected 299 geomagnetic storms
in differen t varying range of hori zontal componen t of
earth ' s magnetic field (H) with Ap2:20 and Ar indi ces
arc prese nted in Tables I and 2, respectively.
INDIAN 1 RADIO & SPACE PHYS . AUGUST 2002
192
Tab le 1- Classification of geomagnetic storms on the basis o f
H co mpo nent with Ap 2: 20
Category
Severe storm s
Moderately severe
storm s
Moderate storm s
H range
Number of storm s
H 2: 400y
250y < H < 400y
97
74
H ::; 250y
128
Table 2 - Class ificat ion o f geo magneti c storms on the basis of
Ap index during th e peri od 1978- 1999.
Category
Intense
Major
Min or
Planetary index
Ap range
No. of geo magneti c
storm s
A 1, 2: 100
50< AP < !00
20 $ A P$ 50
14
77
208
The yearly occ urrence of geo magnetic storms with
Ap:0::20 and th eir assoc iation with various phases of
sunspot cyc les are shown in Fig. 1. A close
between
yearly
occurrence
of
assoc tatt on
geomag neti c storms and sunspot cycle is ap parent
from Fig. I. Somehow, a peculi ar res ult is also
observable during the year 199 1, i.e., the SSNs
decrease slowly with rapid decrease in th e num ber of
sto rm s. No definite trend is establi shed between th e
SS s and the GMSs for thi s case. Solar output
in1lucnces the earth 's magnetic field and produces
geo magnetic disturbances at vari ous locations. These
disturbances, on the basis of daily average va lue of
Ap:0:: 20, are being examined and 1888 dis turbed days
are fo und during th e period 1978-99. The yearl y
occurrence of disturbed days and their variation wi th
sun<>pot cycle are presented in Fi g. 2. No distinct
assoctattolt1 between yearly occurrence of disturbed
days and sunspot cycle is observed from thi s analysis.
It is not always necessary that the number of disturbed
days are maximum during the solar maximum period .
A peculiar result has been observed during the years
1982 and 1993 when disturbed days have increased
significantly, whereas, SSNs have decreased. A
frequency occurrence histogram of monthly
distribution of number of disturbed days with Ap :0:: 20
during the period 1978-99 has been shown in Fig. 3. It
is apparent from Fig. 3 that the maximum number of
disturbed days have occurred during the months of
March-April and September-October. It is also
observed that these di stributi on- show a cyclic
variation having two peaks durin g the solar cycles 2 1,
22 and 23. Further, monthly di stributi on of disturbed
days on year-to-year basis during the peri od 1978-99
has been plotted in Fig. 4. A recurrent/periodic
occutTing tendency of disturbed days with seasons is
clearly seen from Fi g. 4.
All those geomagnetic storms with AP :0:: 20 are
co nsidered during th e period 1978-93 to in ves ti gate
their association with solar features. The association
of geomagnetic storms with importance of Ha so lar
nares and X-ray solar flares during the peri od 197893 has been show n in Fi g. 5 [(a) an d (b)]. From Fig.
5(a), it is apparent that out of 106 GMSs, 83.5 % of
geomagnetic sto rms are associated with Ha so lar
nares of importance :s; IN. Fun er, it is evident
from Fig. 5(a) that maximum number of GMS s arc
associated with Ha solar flares of importance SF
R 0 V V. JABALPUR
R D V V. JABALPUR
I
160
>-
()
z
w
(f)
aw
w
m
::;;
a::
:::J
...a::w
:::J
z
>-
u
z
w
0
a.
(f)
a::
a::
z
:::J
:::J
(f)
()
~ 120~
0
0
w
CD
a::
0
I
0
>(f)
...00
- 120 ~
I
:::J
80r
::0
::;
::0
z
>-
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I~
UJ
CD
z
UJ
CD
1\ ;
a::
::;
u
I
f
a.
-80 <f}
z
::0
<f}
40
40~
7B
82
86
90
94
~
98
YEARS
YEARS
Fig . I - Occurn.:nce freque ncy o f the gc <> lnag nct ic ston m.
(hi stographicall y) and sun spot numbers fo r the p..: ri ocl 1978- 1999
Fig. 2 - Piots show ing th e nu mber o f d is:urbcd da ys with Ap 2:
20 (h istographicall y) and sun spo t number-; fo r th e peri od 19781999
SANTOSH KUMAR & Y ADA V : SOLAR CAUSES OF GEOMAGNETIC STORMS
R D V V, JABALPUR
I
I
I
J
I
I
I
I
I
I
I
I
200 -
-
160 r-
-
(/)
><{
0
0
w
co
a: 120
:::J
f-
-
(solar flare faint) and SN (solar flare normal); and no
significant correlation between magnitude of GMS
and importance of Ha solar flare is observed. Further,
it is apparent from Fig. S(b) that out of 70
geomagnetic storms, 85 .7% of GMSs are associated
with X-ray solar flares of importance <lN . Further, it
is quite evident from Fig. S(b) that maximum number
of GMSs are associated with X-ray solar flares of
importance SF and SN. Again, no significant
correlation between magnitude (intensity) of
r-
-
193
R D V V, JABALPUR
r-
(/)
0
LL
0
a:
-
-
40 -
-
80
w
co
2
:::J
z
0
_L
Jan Mar May
Jul
Sep Nov
u>z
w
::)
MONTHS (1978-99)
aw
0:::
LL
Fig. 3- Histog ram s sho win g th e mo nthly average o f the numbe r
of di sturbed days with Ap 2: 20 fo r th e entire period fro m 1978-99
R 0 \1 V, JABALPUR
24
w
u
z
w
0:::
0:::
::)
u
u
0
20
"'>-
<3 16
0
w
a:
0
::>
ti
5
12
1978
1982
1986
1990
1994
1998
YEARS
Fi g. 4- Plo ts show ing th e yea r-to-year di sturbed day s with Ap ;:::
20 for the e ntire pe ri od 1978-99
FLARE IMPORTANCE
Fig. 5 - Hi stogra ms sho win g the occurre nce freque ncy of the
impo rtance o f (a) Ha sola r fl a res and (b) X-ray so lar fl ares fo r the
period 1978-93
194
INDIAN J RADIO & SPACE PHYS. AUGUST 2002
geo magnetic storm and importance of X-ray solar
flares has been observed. Sometimes, the X-ray solar
flares of lower importance are also able to produce
GMSs. This may be due to X-ray burst (11, IV radio
burst), larger size and more active National Oceanic
and Atomospheric Administration (NOAA) region,
more duration and lower helio-latitt.ide I longitude of
X-ray solar flares. Actually, solar flares of higher
importance are able to produce fast IP shocks in
interplanetary medium which cause the large
geomagnetic storms. The magnitude of GMSs is
associated with two kinds of IP shocks known as fast
and slow IP shocks. These fast and slow IP shocks are
associated with different properties of solar flares, e.g.
occurrence duration, area, NOAA region and location
(helio-longitude/latitude) of solar flares. In that case,
sometimes solar flares of lower importance which are
associated with fast shocks, may produce more
intense geomagnetic storms. But, no significant
correlation between magnitude of GMSs and
importance of solar flares is observed. A frequency
occurrence hi stogram of GMSs with helio-latitude I
longitude of Ha solar flares has been plotted in Fig. 6
[(a) and (b)]. From Fig. 6(a), it is evident that 52.8%
geo magnetic storms is produced by southern Ha solar
flares and most effective latitudinal zone for
producing geomagnetic storms is (0-40) 0 S. At the
helio-latitude in the range (0-30tN and (0-30) 0 S,
there is a concentration of 94.3% of the total Ha solar
flares associated with GMSs and no storm is produced
by Ha solar flare beyond 40°N and 40°S. Further, it is
quite apparent from Fig. 6(b) that 51.8% geomagnetic
storms is produced by western Ha solar flares and
48 . 1% is produced by eastern Ha solar flares. Again,
72.6% geomagnetic storms is produced by Ha solar
flares of helio-longi tude between 0 ° and 60°E and
between ooand 60°\V. Thus, it may be deduced that
Ha solar flares occurred within lower heliographic
latitude produce maximum number of GMSs.
Therefore, the Ha solar flares occurred within lower
heliographic latitude are able to produce a
configuration of closed magnetic field regions, which
may be associated with IP shocks in the interplanetary
medium leading to cause GMSs on the earth. On the
higher heliographic latitudes, Ha solar flares have not
occurred in association with GMSs.
The association of GMSs with different
heliographic zones of X-ray solar flares is also
investigated. Frequency occurrence histogram of
R D V V, JABAL PUR
-T
I
I
I
I
I
'
I
(a) FLARE'S HELIOLAT:TUO E
I
I
-
30 ~
Nor th
South
20
>-
-
10 1-
u
z
w
LL
n
n
:J
530:: 0
:
w
u
z
w
:
(b) FLARE'S HELI OLONGITUOE
0::
g5 16
u
East
u
0
West
~
-
4 1-
-
12
8
0
.nl .~1
lo
II
lr
~ ,rl.n ~n.n~
II
Fig. 6- Histograms showing the occurrence frequency of the Ha
solar flares with (a) Helio-latitude and (b) Heli o- longit ude during
the period 1978-93
GMSs with helio-latitude/ longitude of X-ray so lar
flares during the period 1978-93 h ve been plotted in
Fig. 7 [(a) and (b)]. From Fig. 7(a), it is observed that
64.3% and 35.7% GMSs are produced by northern
and southern X-ray solar flares, respectively, in the
heliographic latitude range (0-40) 0 N and (0-40t S. At
helio-latitUJde in the ranges (0-40) 0 N and (0-40t S,
there is a concentration of 100% of total X-ray solar
flares with GMSs and no storm is produced by X-ray
solar flare beyond 40°N or 40°S. Further, it is quite
apparent from Fig. 7(b) that 51.4% and 48.5 % GMSs
are produced by western and eastern X-ray so lar
flares, respectively. It is further observed that 82.8%
GMSs are produced by X-ray solar flares in heliolongitude between oo and 60°E and between 0° and
60°\V. Thus, it may be inferred that X-ray solar flares
which occurred within lower heliographic latitude
may produce larger number of geomagnetic storm s.
Frequency occurrence histograms of GMSs with
heliographic latitude/longitude of active prominences
SANTOSH KUMAR & Y ADA V : SOLAR CAUSES OF GEOMAGNETIC STORMS
195
R D V V, JABALPUR
R D V V. JABALPUR
(a) A PDF'S HELIOLATITUDE
(a) FLARE'S HELIOLATITUDE
15
North
South
South
North
10
i:iz
w
::J
~ 4
0::
l1.
w
u
z
(b) FLARE"S HELIOLONGITUDE
w
0::
0::
::J
East
West
u
g 12
(b) A P 0 F 'S HELIOLONGITUDE
East
10
West
Fig. 7 - Hi strograms showing the occ urrence frequency of X-ray
solar flares with (a) He lio-latitude and (b) Helio-longitude during
the period 1978-93
Fig. 8 - Histrograms showing occurrence frequen cy of the APDFs
with (a) Helio-latitude and (b) Helio-longitude during the period
1978-93
and disappearing filaments (APDFs) during the period
1978-93 have been plotted in Fig 8[(a) and (b)]. It is
observed from Fig. 8(a) that 54.7% geomagnetic
storms are produced by northern APDFs and the most
effective helio-latitudinal zones for producing GMSs
lie between 0° and 40°N. At helio-latitude (0-40) 0 N
and S, there is a concentration of 92.8% of total
APDFs that are associated with GMSs and no storm is
produced by APDFs beyond 50°N and 60°S. Further,
it is quite evident from Fig. 8(b) that almost equal
number of GMSs are produced by each eastern and
western APDFs solar features. It is observed from
Fig. 8(b) that 55.9% GMSs is produced by APDFs in
heliographic longitude range 0-60°E and 0-60°W.
Thus, it may be inferred that the APDFs, occurred
within lower heliographic latitudinal zones, produce
maximum number of geomagnetic storms.
solar flares, APDFs and CMEs, respectively. It is
observed that maximum number of GMSs seem to be
associated with solar flares (Ha and X-ray solar flares),
because solar flares occur more frequently than
prominence eruptions. This agrees with the Garcia and
Dryer 10 results and is inconsistent with the results of
Hewish and Bravo 19 and Webb25. The prominence
eruptions have occurred on an average of about
0.55/day, during 1976-1979, while solar flares have
occurred at the rate of 1.19/day during the period 19781979(Ref. 13). Associations of GMSs with different
combination of solar features, i.e., Ha, X-ray solar flares,
APDFs and CMEs have been shown in Fig. 9. It is also
observed from Fig. 9 that six GMS are not associated
with any solar features, revealing that some solar
phenomena occurred on the back portion of solar disc
which could not be seen during the observation, have
caused the geomagnetic storms.
Finally, the association of significant GMSs (216)
with Ap ~ 20 which are investigated during the period
1978-93 with various solar activities is discussed.
Association of these GMSs during the said period has
been plotted using Venn diagram in Fig. 9. It is quite
clear from Fig. 9 that I 06, 70, 84, 30 geomagnetic
storms are associ ated with each Ha solar flares , X-ray
4 Conclusions
From the present statistical analysis of the GMSs for
the period 1979-99, the following conclusions may be
drawn:
INDIAN 1 RADIO & SPACE PHYS. AUGUST 2002
196
R D V V, JABALPUR
The authors are also thankful ro the anonymous
referees for their valuable suggestions.
Referenc~es
I Arnoldy R L, J Geophys Res (USA!. 76 ( 1971) 5 189.
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Stream, edi ted by 1 8 Zirker (Bou lde r, Co lorado : Colo.
Assoc. Univ. Press, USA), 1977, p. 225.
1 : H,SOLAR FLARES (106)
2 : X-RAY SOLAR FLARES'(70)
3 : APDF s (84)
4 : CMEs (30)
5 : UNKNOWN (06)
6 : H,. X-RAY (66)
7 : APDF s. CMEs (24)
8 H0 , CMEs (29)
9 : H,,X-RAY, APDFs (57)
10 H0 ,X-RAY, CMEs (27)
11 · H0 ,X-RA Y, APDFs . CMEs (2 3)
Fi g. 9- Venn diagram showing the solar origin of 216
geomag netic sto rms with Ap ~ 20 during the period Nov 1978Dec 1993
(i)
There is no significant correlation between yearly
distribution of GMSs and the number of disturbed
days with su nspot numbers during different solar
cycles.
(ii)
The number of disturbed days vary with seasons.
Further, year-to-year distribution of disturbed days
shows a cyclic variation having two peaks during
each solar· cycles.
(iii) The maximum number of geomagnetic storms are
associated with solar flares of lower importance, i.e.,
SF during the period 1978-93. Thus, the solar flares
with lower importance in association with some
other specific properties, i.e. position, duration,
region, etc., may also cause GMSs.
(iv) Solar features, e.g., Ha, X-ray solar fl ares and
APDFs mostly occur within lower heliographic
latitudinal zones and produce maximum number of
geomagnetic storms .
Acknowledgements
The authors are thankful to various experimental
groups for providing the data. The SWP and IMF data
prov ided by Prof. J H King are highly acknowledged.
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